Definition
A Solid-State Relay (SSR) MPC555LFMZP40 is a modern, contactless switching device composed entirely of solid-state electronic components. Unlike traditional electromechanical relays, SSRs use semiconductor devices such as transistors and thyristors to perform switching operations without physical contacts. This enables SSRs to switch circuits on and off without sparks or mechanical wear. SSRs are often referred to as "contactless switches" because of their ability to switch without the physical contact that creates sparks and wear.
SSRs are designed as four-terminal devices, with two terminals for input control and two for output control. Their lack of moving parts results in faster switching speeds, longer lifespan, and greater resistance to external interference.
Working Principle
A Solid-State Relay (SSR) operates by using semiconductor components to perform the switching function instead of mechanical contacts. When an input control signal, typically a low voltage, is applied to the control terminals, it activates an optocoupler or a light-emitting diode (LED) within the SSR. This optocoupler isolates the low-voltage control circuit from the high-voltage load circuit. The light emitted by the LED triggers a photodiode or a phototransistor, which then activates a semiconductor switch, such as a thyristor, triac, or power transistor, connected to the output terminals.
The semiconductor switch MPC555LFMZP40 allows or interrupts the flow of current in the load circuit, effectively switching it on or off. The switching is done by solid-state components without any mechanical movement. And the lack of moving parts also means there are no sparks or contact erosion, reducing maintenance and improving performance in environments with frequent switching or high-power demands.
Features
Fully sealed injection method
Zero-crossing trigger technology
Can reduce the voltage rise rate and current rise rate
Short switching time, about 10ms, can be used in high-frequency occasions
Optoelectronic isolation is used between the input circuit and the output circuit, and the insulation voltage is above 2500V
Very low input power consumption
Compatible with TTL, COMS circuits MPC555LFMZP40
Protection circuit at the output end
Strong load capacity
Advantages & Disadvantages
Advantages
Able to work in high impact and vibration environments
High sensitivity
Low control power
Good electromagnetic compatibility
Fast switching speed.
Low electromagnetic interference
Disadvantages
Cannot achieve ideal electrical isolation
High cost
Poor temperature characteristics, anti-interference ability and radiation resistance
High sensitivity to overload
Exists on-state voltage drop
Applications
Control systems
Computer peripheral interface equipment
Thermostatic systems
Motor control
CNC machinery
Remote control systems
Industrial automation devices
Lighting control systems
Instruments and meters
Automatic fire protection
Chemical and coal mining
Installation and Operation of the Solid State Relay
Installation
Solid-state relays (SSRs) come in various types suited for different installation methods. The horizontal (W) and vertical (L) types are compact and suitable for direct soldering onto printed circuit boards. The L2 type can be either soldered or plugged into circuit boards. When using SSRs with small current specifications for printed circuit boards, solder at temperatures below 250℃ for less than 10 seconds to avoid damage.
To manage heat, especially under higher ambient temperatures, it is recommended to use the relay at half its rated current. For larger SSRs (K and F types) that require heat sinks, make the contact surface flat and coated with thermal grease to minimize thermal resistance. For high current connections, use cold-pressed welding pieces to reduce contact resistance.
Operation
SSRs can be controlled by various input methods, including resistive, constant current source, and AC input. The 5V TTL level resistive input type is commonly used. When using other control voltages, use appropriate current-limiting resistors. The SSR input is current-driven, and once the opto-isolated triac is fully on, it triggers the power triac. Insufficient or ramped trigger voltage may leave the power triac on the verge of conduction, risking damage to the main load circuit.
To prevent excessive input voltage or current, consider using series voltage-dividing resistors or parallel shunt resistors at the input. In practice, it is also important to have a stable control signal and load power supply with fluctuations not exceeding 10%.
Precautions
If you want to prevent misoperation and loss of control, install SSRs away from sources of electromagnetic and radio frequency interference. SSRs can also emit interference, especially when switching loads. Incandescent lamp resistive loads generate less interference, particularly when using zero-crossing SSRs that switch near zero voltage points. To reduce interference, use series inductance coils with the load and avoid crossing signal lines with power lines.
FAQs
Why do solid-state relays heat up?
When solid-state relays are working normally, there is a certain amount of power loss on their internal chips. This power loss is mainly determined by the product of the output voltage drop of the solid-state relay and the load current, and is consumed in the form of heat.
How to use a proper heat sink?
Except for the solid-state relays with a rated current of 1-5A that are directly mounted on the printed circuit board, the rest should be equipped with a proper heat sink, and thermal grease should be applied between the SSR base plate and the heat sink, and the two should be in close contact and tightened with screws.
How to protect SSR?
Overcurrent protection. SSR is a semiconductor power device, which is extremely sensitive to temperature changes. Overcurrent will damage SSR, and fast fuses are usually used.
Add RC absorption circuit. Adding RC circuits not only prevents overvoltage, but also helps improve dv/dt.
Overheat protection. If SSR overheats, the characteristics will decrease, and it may lose control or cause permanent damage. It is recommended to install a temperature control switch near the bottom plate of SSR, and the temperature control point should be between 75 and 80℃.
Connect inductor L in series with inductive load. In inductive load, SSR is usually damaged due to high current change rate di/dt.